1. Field of the Invention
The present invention relates to a magnetic resonance (MR) apparatus, and more particularly, to a magnetic resonance apparatus capable of acquiring magnetic resonance data of an object under examination at a very high speed and at a very high resolution.
2. Description of the Related Art
Magnetic resonance apparatuses chiefly adapted for magnetic resonance imaging generally comprise: a static magnetic field coil system for generating a static magnetic field; a gradient coil system for generating linear gradient fields having strength distributions in mutually orthogonal directions; a high-frequency or radio-frequency (RF) coil system, or a probe coil system for producing a magnetic resonance phenomenon and detecting a magnetic resonance signal produced by the magnetic resonance; a drive circuit for driving the coil systems; and a control and processing device for controlling the drive circuit in a predetermined manner and processing the magnetic resonance signal detected through the high frequency coil system for the purpose of magnetic resonance imaging.
Each of the coil systems is usually comprised of a plurality of coils according to the apparatus scale and the coil geometry. The gradient coil system generates a slice gradient magnetic field Gs for selecting a slice of an object under examination, and encode and readout gradient magnetic fields Ge and Gr for causing two-dimensional position information of a source of magnetic resonance signal in a plane of the slice to be included in the magnetic resonance signal. The encode gradient magnetic field Ge is used at a time of phase encoding of the magnetic resonance signal and the readout gradient magnetic field is used at a time of reading out the magnetic resonance signal. The magnetic resonance signal comes to have position information (information about one direction) due to the encode gradient magnetic field Ge. Likewise, the magnetic resonance signal comes to have position information (information about another direction) due to the readout gradient magnetic field Gr. The control of currents applied to the gradient coil system through the drive circuit by the control and processing circuit will allow the two-dimensional imaging of the specific slice of the object.
The probe coil system adapted for transmitting and receiving the high frequency signal may be set to a suitable shape and size in accordance with the object under examination and/or a portion to be measured. As the probe coil system, a coil system adapted for transmission and reception or a combination of a coil system adapted for transmission only and a coil system adapted for reception only may be used. In particular, in order to image a local portion of the object, such a surface coil as shown in FIG. 1 may be used as the probe coil system, to transmit and receive or receive a high frequency signal.
For local spectroscopy for obtaining a magnetic resonance spectrum of a local portion of an object under examination, the depth resolved surface coil spectroscopy (DRESS) method has been proposed which combines the surface coil and the selective excitation method.
As high-speed imaging methods for acquiring magnetic resonance data at high speed the echo planar method and the fast Fourier method have been proposed. An example of the timing diagram of high frequency pulse RF, slice gradient field Gs, readout gradient field Gr, and encode gradient field Ge in the pulse sequence of the echo planar method is shown in FIGS. 2A-2D. Further, an example of the timing diagram of high frequency pulse RF, slice gradient pulse Gs, readout gradient pulse Gr and encode gradient pulse Ge in the pulse sequence of the fast Fourier method is shown in FIGS. 3A-3D.
In existing magnetic resonance apparatus, gradient coils apply gradient fields to the entire body of an object or a considerable proportion of the body irrespective of the size of a portion to be measured. For this reason it is necessary to always supply the gradient coils with large currents. However, in the case of imaging of a local portion, it is not practically necessary for the gradient coils to apply gradient fields to a large proportion of the body. As long as the gradient fields exist only within the sensitive range of the probe coil, the imaging is not at all affected. On the other hand, the imaging of a local region corresponding to a local portion at high speed and high resolution requires gradient magnetic fields of very great amplitude. Further, the gradient fields must be switched quickly. However, with the existing gradient coil systems, the coils themselves are large in size and their inductance is large. Therefore, even if a special power supply for gradient fields is used, it is not easy to provide gradient fields of sufficiently great amplitude, and it is not possible to obtain switching characteristics which allow sufficiently quick switching of gradient fields. Further, to image a local region at high speed and high resolution, an operation for selecting the region to be imaged should be made rapid and easy.